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Halides are inorganic compounds in which one part is a halogen atom with negative charge, and the other part is an element or radical that is more electropositive than the halogen. Metal halides are used in high-intensity discharge lamps. Metal halide lamps are also commonly used in greenhouses or in rainy climates to supplement natural sunlight. Silver halides are used in photographic films and papers. Halides are also used in solder paste, commonly as a Cl or Br equivalent.
Some, metal halides such as sodium chloride are ionic, while others are covalently bonded. Covalently bonded metal halides may be discrete molecules, such as uranium hexafluoride, or they may form polymeric structures, such as palladium chloride. Some metal halides also act as Lewis acids. Ferric and aluminum chlorides are efficient and often are used as catalysts for the Friedel-Crafts reaction. Chloroplatinic acid (H2PtCl6) is an important catalyst for hydrosilylation. Titanium tetrachloride (TiCl4) is a popular reagent for the Zeigler Natta polymerization of olefins. Chlorides of palladium are utilized in organic coupling reactions. Metal halides are employed as available precursors for other inorganic compounds.
Calcium fluoride or fluorspar (CaF2) is by far the most important compound as it is the only large scale source of fluorine. CaCl2 is widely used for melting ice on roads, particularly in very cold countries. Anhydrous calcium chloride has a strong affinity for water and is used as a popular dehydrating agent. Anhydrous MgCl2 is used in the electrolytic extraction of magnesium. Samarium iodide is often used in organic synthesis as a mild one electron reductant.